The Russian city of Chelyabinsk had a rude awakening early on February 15th when a meteor exploded overhead. The blast wave shattered windows and injured an estimated 1,000 people. Based on preliminary evidence from infrasound stations built to monitor nuclear tests, this looks to be an historic event.

“We know that the energy of the explosion was about 300 kilotons of TNT equivalent.”

Margaret Campbell-Brown, a professor who studies meteoroids at the University of Western Ontario.

“So it was a very, very powerful explosion. It was the biggest explosion from a meteor that we’ve seen in the atmosphere since 1908, since the 1908 Tunguska impact.”

The cause appears to be an asteroid, which Campbell-Brown estimates was 15 meters across. Objects of that size are expected to hit Earth only once every half-century or so. And impacts over cities of more than one million people such as Chelyabinsk are rarer still.

“When you consider all the areas of the Earth that are uninhabited—the oceans, the ice caps, the deserts and so on—it’s very surprising that this happened over such a populated area. Very unlucky.”

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